She does mention “the train back home again,” which cuts the unexplained portion of the three-hour period from 5pm (“at five I must have left”) to 8pm (“opened my front door”) down to three hours minus commuting time. 

You’re right,  the sequence of events as given suggests that she passed the Chinese place “along the way” from her front door to her living room, where she would then consume her dinner “watching something on TV.” Perhaps such takeout facilities in the hallway are standard in middle and upper class Swedish homes, similar to having a washer & dryer in the apartment. I’m not an expert on the Scandinavian way of life. 

I suppose the confusion could have been reduced by replacing “and” with “having”: 

I must have opened my front door at eight o’clock or so
Having stopped along the way to buy some Chinese food to go

That, of course, would have been one syllable too many. 

Still, I’m not quite sure what might be the proper way to translate such a construction into the past-perfect tense, as would be indicated here, assuming she purchased her dinner prior to opening her front door, as the lyricist most likely meant — so before she “must have opened” her front door, she must have had (???) stopped along the way to buy some Chinese food to go? 

If the song were written in simple past tense in lieu of the speculative  ”must have” mode (a subjunctive of some sort, I guess), she could simply sing: 

I opened my front door at eight o’clock or so
I’d stopped along the way to buy some Chinese food to go

Here, the past-perfect tense renders the sequence of events perfectly clear: first she bought her food, then she arrived home, even though the two actions are being mentioned in reverse chronological order. 

But if she starts with “I must have opened my front door,” then I have no idea how to correctly communicate in the following line that she must have bought some Chinese food prior to having opened her front door, let alone without inserting a host of extra syllables (e.g., the word “before”) that wouldn’t fit the music.

So in your little puppet re-enactment there, the poor woman actually misses her morning train and reads the paper as she walks (!) into town, which may account for her arriving at her desk 15 minutes late, at a quarter after nine. 

Of course, having left her house at 8am, presumably planning to make her desk at 9am by taking the train, yet arriving no more than 15 minutes late after having traversed the entire distance on foot instead, it would mean that the morning trains in that part of the world weren’t exactly of the high-speed variety. 

But it also means that her door-to-desk commute in the morning would have taken only an hour — as opposed to an hour and 15 minutes — had she managed to catch her train. Therefore, her uninterrupted evening desk-to-door commute, where she did succeed in catching the train, would have taken no more than an hour as well. This then would drive up the unexplained time period from 1h 45min to an even two hours. 

You’re correct in observing that if she stopped on her way to the train station in the morning to purchase the morning paper (assuming she didn’t have home delivery), the law of workday symmetry (which, incidentally, also says that morning commute equals evening commute) suggests  that she would have bought the evening paper en route to her evening train, a transaction that would necessarily have taken up some time, thus shortening the unexplained time gap as follows: 

1h 45 minutes (or 2 hours, if calculated based on your scenario where she missed her morning train every day) minus time spent buying Chinese food to go minus time spent purchasing the evening paper 

(Having worked in the restaurant business in the U.S. for many years, I confess that the nuanced distinction regarding promptitude, should one exist at all, between “takeout” and “to go” has escaped me. As far as I’m aware, these two terms are being used interchangeably.)

Now, I’m no expert on Scandinavian economy during the early 1980s, but if the region was going through a Great Depression at the time, it would explain unusually long lines and wait times for basic goods, such as food and newspapers, in which case buying a paper and some dinner to go could easily have taken up two hours. (If her morning paper was delivered to her home, it would explain why she at least didn’t have to line up for an hour each morning to buy it. This, of course, would violate the law of workday symmetry, but there are exceptions to every law.) 

Unfortunately, I cannot find any evidence of such economic woes in the region at that time, which renders the Great Swedish Depression theory somewhat far-fetched.  

Although Dallas did indeed air on Fridays in the U.S., it aired on different days in different countries. In my native Austria, for instance,  Dallas day was Tuesday (“Montag, Dallastag, Mittwoch, Donnerstag, …”). In Sweden, who knows, it may have aired on Mondays or Wednesdays. 

Besides, she only mentions Dallas as an example to illustrate the general type of shows she would gravitate toward at the time, just as she mentions Marilyn French “or something in that style” to illustrate the type of material she would read. She doesn’t say it was necessarily that particular show or that particular author she consumed on the very day (or night) before “you” came. The song merely describes her typical day at that stage in her life. 

Of course, there are a million things she could have done during those unexplained two (or almost-two hours) of her typical evening. Perhaps she went to the tanning salon to combat her congenital Scandinavian pallor, or perhaps she had a habit of stopping by for a chat with her best girlfriend who worked the night shift at the local Knäckebrot factory. The possibilities are endless.

The point is, the song itself is silent on the two hours before she opened her front door at eight, whereas it minutely chronicles her whereabouts for the rest of the day. 

In fairness, the song features a 35-second-long instrumental passage (roughly between time 2:30 and 3:05) before the singer comes back with “I must have opened my front door at eight o’clock or so” — could it be that the clue to the lady’s activities during those missing hours can be gleaned from the music? Does the music tell us what she did right before she got home at night?  

If so, I find myself at a loss to translate it.